Non-crimp fabrics made from reinforcing fibers or yarns have long been known on the market. For this purpose multiaxial non-crimp fabrics are often used that have a structure made from a plurality of superimposed fiber layers, wherein the fiber layers consist of sheets of reinforcing fibers arranged parallel to each other. The fiber layers are superimposed such that the reinforcing fibers of the layers are oriented parallel to each other or alternately crosswise. The angles are virtually infinitely adjustable. Usually however, for multiaxial non-crimp fabrics angles of 0°, 90°, plus or minus 25°, plus or minus 30°, plus or minus 45°, or plus or minus 60° are set and the structure is selected such that a symmetrical structure with respect to the zero-degree direction results.
Fabrics such as the cited multiaxial non-crimp fabrics can be used due to their structure especially for the manufacturing of complex constructions. The non-crimp fabrics are thereby laid without matrix material in a mold and are adapted to the contours thereof. By this means, a so-called preform is obtained, into which the matrix material required for producing the composite component can subsequently be introduced via infusion or injection, or also by the application of vacuum. Known methods are the so-called liquid molding (LM) method, or methods related thereto such as resin transfer molding (RTM), vacuum assisted resin transfer molding (VARTM), resin film infusion (RFI), liquid resin infusion (LRI), or resin infusion flexible tooling (RIFT).
Fiber composite components produced using non-crimp fabrics of this type made from reinforcing fibers are suited in a superb way to directly counteract the forces introduced from the directions of stress of the component and thus ensure high tenacities. The adaptation in the multiaxial non-crimp fabrics with respect to the fiber densities and fiber angles, to the load directions present in the component enables low specific weights.
The superimposed fiber layers can be connected and secured to each other via a plurality of sewing or knitting threads arranged adjacent to each other and running parallel to each other forming stitches, such that the multiaxial non-crimp fabric is stabilized in this way. The sewing or knitting threads thereby form the zero-degree direction of the multiaxial non-crimp fabric. Multiaxial non-crimp fabrics of this type can be produced e.g. by means of standard warp knitting looms or stitch bonding machines, for example by means of the LIBA machines or Karl Mayer machines known to a person skilled in the art. Multiaxial non-crimp fabrics connected by means of sewing or knitting threads and the manufacture thereof are described for example in DE 102 52 671 C1, DE 199 13 647 B4, WO 98/10128, or EP 0 361 796 A1.
EP 1 352 118 A1 discloses multiaxial non-crimp fabrics, for which the layers of the reinforcing fibers are held together by means of fusible sewing yarns which enable a good shapeability of the multiaxial non-crimp fabrics above the melting temperature of the sewing threads and a stabilization of the shape during subsequent cooling. Sewing threads made from thermoplastic polymers such as polyamide or polyester are often used, as is disclosed in EP 1 057 605 for example.
A preproduct for a composite preform is described in US 2005/0164578, which preproduct has at least one layer made from a reinforcing fiber woven fabric and in which fibers for stabilization are integrated in at least one of the layers, which fibers stabilize the preform when they are subjected to increased temperatures and which fibers dissolve in the matrix resin introduced later for the production of the composite component. WO 02/16481 also discloses structures made from reinforcing fibers for e.g. preforms, wherein the structures contain flexible polymer elements which are e.g. introduced in the form of fibers between the reinforcing fibers or as sewing threads that connect the reinforcing fibers with each other. The flexible polymer elements comprise a material that is soluble in the hardenable matrix material used.
According to DE 198 09 264 A1, adhesive non-wovens made from thermoplastic polymers can be inserted between the layers, which are sewn to each other and made from reinforcing fibers, of the fiber fabric arrangements disclosed therein. Due to these meltbonded non-wovens, the fiber fabric arrangements can be shaped in a simple way, when heated above the melting temperature of the polymer forming these non-wovens, into three-dimensional structures which maintain their shape after cooling practically without reset forces.
Also, sometimes randomly-laid fiber mats or non-wovens, or staple fiber fabrics or mats, are to some extent laid between the layers made from reinforcing fibers in order to improve e.g. the impregnability of the non-crimp fabrics or to improve e.g. the impact strength. Multiaxial non-crimp fabrics of this type having mat-like intermediate layers are described for example in DE 35 35 272 C2 or US 2007/0202762, wherein values of mass per unit area between 100 and 1200 g/m2 are disclosed for the non-wovens or mats in DE 35 35 272 C2 and values of mass per unit area of 40 g/m2 to 161 g/m2 in US 2007/0202762.
EP 1 473 132 has as its subject matter multiaxial non-crimp fabrics and a method for the production of said multiaxial non-crimp fabrics as well as the preforms produced from the multiaxial non-crimp fabrics. The multiaxial non-crimp fabrics described therein have intermediate layers made from thermoplastic fibers between the layers made from reinforcing fibers laid unidirectionally, wherein the intermediate layers can be non-wovens made from bicomponent fibers or hybrid nonwovens made from different fibers mixed together. The polymer forming the intermediate layers should be compatible with the matrix resin injected later in the preform. In particular, it is explained that the intermediate layers should be permeable for the matrix resin during the resin infusion and should secure the reinforcing layers during and after the resin infusion. In the case of use of epoxy resins, the non-wovens are made from polyamide fibers. The nonwovens can be connected to the layers made from reinforcing fibers via knitted stitches or via meltbonding.
EP 1 772 258, too, discloses a laminate structure for the production of fiber reinforced plastic parts. These laminate structures have a non-woven layer with volumes of mass per unit area from 100 to 500 g/m2 as a core layer and at least one cover layer made from reinforcing fibers. The non-woven layer is for example a fiber mixture made from support fibers and thermoplastic binding fibers and the melting point of the binding fibers is lower than that of the support fibers. During heat treatment at a temperature above the melting point of the binding fibers and below the melting point of the support fibers, a thermal reinforcing of the non-woven layer is achieved according to EP 1 772 258 and thereby a higher internal strength and dimensional stability of the non-woven layer. At the same time, the non-woven layer guarantees a high permeability during the infiltration with matrix resin.
US 2008/0289743 A1 discloses multiaxial non-crimp fabrics made from alternatingly arranged layers of reinforcing fibers and non-wovens made from thermoplastic fibers as intermediate layers, wherein the intermediate layers are arranged between the reinforcing layers and are connected to the same via knitted stitches or meltbonding. In one embodiment, the non-wovens can be constructed from two or more materials and are thus hybrid nonwovens or bi-component or tri-component non-wovens, etc. According to a particular embodiment, a non-woven can be made from core/sheath fibers with a core made from polyamide and a sheath made from polyurethane. The non-wovens additionally serve to secure the unidirectionally arranged reinforcing fibers and to guarantee the resin flow during the resin infiltration. In a preferred embodiment, the curing should take place at temperatures below the melting temperature of the thermoplastic fibers of the intermediate layer.
A disadvantage of the previously described composite constructions of the prior art is the relatively high proportion of material that does not consist of reinforcing fibers and thus does not contribute to the strength of the resulting component. The matrix material must be referred to the total amount of reinforcing fibers and non-woven, such that, in relation to the component volumes, a lower proportion of reinforcing fibers in the component and thus in lower strength results.
EP 1 705 269 discloses a thermoplastic fiber material made from a polyhydroxy ether, which e.g. can be used in the case of multiaxial non-crimp fabrics made from reinforcing fibers, e.g. as a non-woven between the layers made from reinforcing fibers. Under application of heat, the polyhydroxy ether material becomes viscous and sticky, such that a fixation of the reinforcing fibers in a defined geometric arrangement can be achieved prior to their embedding in the matrix. The polyhydroxy ether fiber material then later dissolves completely in the matrix material at a temperature above its glass transition temperature.
Non-crimp fabrics made from a plurality of layers of reinforcing fibers are described in US 2006/0252334, which contain e.g. non-wovens made from polymer fibers between the reinforcing layers to improve the impact strength of the components produced from these non-crimp fabrics. These polymer fibers should thereby be soluble in the matrix resin, by which means according to the statements of US 2006/0252334 a more uniform distribution of the polymer forming these fibers in the resin matrix is enabled in comparison to meltable, insoluble thermoplastics.
Because the polymer fibers for the fabrics of US 2006/0252334 and EP 1 705 269 are soluble in the matrix material and as a result dissolve during the infiltration of the non-crimp fabrics with matrix resin, a secure fixation of the reinforcing layers in this stage of the component production is not sufficiently guaranteed.
There exists therefore a need for non-crimp fabrics, based on reinforcing fibers that have a good drapability and dimensional stability after the shaping into preforms as well as a good permeability during the infiltration of matrix resins. At the same time, the components produced from these non-crimp fabrics should possess high strength properties in particular under application of pressure, and a high impact strength.